The invention relates to a process for the indirect heat exchange of a plurality of gas streams with a heat/cold carrier in heat-exchange blocks in which the gas streams are passed through a multiplicity of heat-exchange passages, with only one of the gas streams being passed through at least one heat-exchange block. In addition, the invention relates to a heat-exchange apparatus for the indirect heat exchange of at least two gas streams with a heat/cold carrier in heat-exchange blocks which have a multiplicity of heat-exchange passages.
In the low-temperature fractionation of air, the feed air to be fractionated must be cooled to the process temperature. This is customarily performed in the main heat exchanger by indirect heat exchange of the feed air with the gas streams produced. The main heat exchanger is generally constructed as a plate heat exchanger which has a multiplicity of heat-exchange passages for the streams to be treated. In air-fractionation plants where large amounts of air are processed, a plurality of such heat-exchange blocks are necessary to process the amounts of air and product. Customarily, in the main heat exchanger, from about 20,000 to 30,000 m3(S.T.P.)/h of air are divided into two blocks.
Customarily, to date, all of the gas streams and the feed stream and if appropriate other streams are passed through each of the individual heat-exchange blocks. If, for example, two air streams of different pressures are fed to an air-fractionation plant and the gaseous products produced are oxygen, pure nitrogen and impure nitrogen, five streams must be passed through each heat-exchange block. Each heat-exchange block must therefore have ten connection ports for these streams, five each for the gas inlet and five for the gas outlet.
Correspondingly, ten apparatuses, termed collector/distributor below, are required in order to distribute the gas streams from the respective inlet port to the assigned heat-exchange passages and, respectively, to combine the gas streams exciting from the heat-exchange passages into the appropriate outlet ports.
The collectors/distributors have been implemented to date by distribution zones integrated into the heat-exchange block. In this distribution zones, at least some of the lamellae (i.e. closely spaced thin plate) fins which separate the individual heat-exchange passages from one another are arranged at an incline, so that the gas flowing in via the inlet port is conducted into the heat-exchange passages or such that the gas stream exiting from the heat-exchange passages is deflected to the outlet port.
The flow conditions are, however, greatly altered in the distribution zones of such collectors/distributors. Firstly, owing to the inclined orientation of the lamellae, a change in flow direction occurs, secondly, the cross-sections of the heat-exchange passages are markedly decreased in the distribution region, as a result of which the velocity of the gas flowing through can he changed. Both effects produce an unwanted pressure drop in the heat-exchange blocks.
DE-A-42 04 172 discloses dividing the main heat exchanger of an air-fractionation plant into a plurality of blocks on the process side, with each product stream produced in the air-fractionation plant being fed via a separate heat-exchange block against feed air. The purpose of the process is to decrease the control requirement for the individual heat-exchange blocks. DE-A-42 04 172, on the other hand, is not concerned with the pressure drop caused by the distribution zones of the blocks and therefore also does not contain any measures which would be suitable for decreasing this pressure drop. The object of the present invention is to develop a process and an apparatus for the indirect heating or cooling of a plurality of gas streams in which the pressure drop in the heat exchanger is as small as possible.
This object is achieved according to the invention by a process of the type mentioned at the outset, in which the heat-exchange passages for the one gas stream of the at least one heat-exchange block end at two end surfaces of the heat-exchange block and the one gas stream is fed to and taken off from the heat-exchange passages of the at least one heat-exchange block via in each case a collector/distributor connected to the heat-exchange block, which collector/distributor extends in each case over the entire end surface of the heat-exchange block.
The inventive heat-exchange apparatus for the indirect heat exchange of at least two gas streams with a heat/cold carrier in heat-exchange blocks which have a multiplicity of heat-exchange passages is distinguished by the face that the heat-exchange passages of a heat-exchange block which are provided for one of the gas streams end at two opposite end surfaces of the heat-exchange block and are each flow-connected to a collector/distributor, the collectors/distributors extending in each case over the entire end surface of the heat-exchange block.
According to the invention at least one gas stream which is to experience as small as possible a pressure drop is passed through a heat-exchange block through which otherwise no other gas streams are conducted. Obviously, through this heat-exchange block, flow one or more heat or cold carriers with which the gas stream exchanges its heat. The heat-exchange passages of this heat-exchange block provided for this gas stream extend from an end side of the block to the opposite end side and run essentially in parallel. At the two end sides at which the heat-exchange passages end in each case a collector/distributor is mounted externally on the heat-exchange block, which collector/distributor covers the entire end surface and has a connection port for the feed line or outlet line. The heat-exchange passages thus pass without cross-sectional tapering into the feed line or outlet line and the flow deflection in the collector/distributor takes place slowly. The pressure drop in the heat-exchange block in the associated collectors/distributors is thus minimized.
By means of the inventive process and the corresponding apparatus, pressure drops in the heat-exchange blocks, measured from the inlet port to the outlet port, of about 70 mbar may be achieved. In comparison, in the conventional heat exchangers in which the distribution and combination of the gas streams between the inlet port and outlet port and the heat-exchange passages take place via a distribution zone which is integrated into the heat-exchange block and has inclined lamellae, a pressure drop of about 100 mbar occurs, if the gas streams are taken off from the low-pressure column at a pressure between 1.2 and 1.8 bar. On the unpressurized side, the invention achieves a reduction in pressure drop of about 30 mbar. This means that the low-pressure streams can be produced at a pressure which is lower by 30 mbar than otherwise. To maintain the heat-exchange conditions in the main condenser it is then sufficient if the air is compressed downstream of the air compressor to a pressure about 90 mbar lower.
Preferably, a separate heat-exchange block is provided for each gas stream. Firstly, this has the above-described advantage of the low pressure drop, secondly the amount of tubing required is decreased. In addition, there is also the reduction in costs of the heat-exchange blocks, since the distribution zones are made considerably simpler. In the customary process in which all gas streams flow through each heat-exchange block, each gas stream requires both on the cold side and on the warm side of the main heat exchanger in each case a manifold line as feed line or outlet line having a plurality of branches to each heat-exchange block. If, in contrast, each gas stream is conducted through a separate heat-exchange block, the branches can be dispensed with and the tubing is considerably simplified.
If the gas rate which is to be conducted via a separate heat-exchange block is so high that it cannot be processed in this block, two or more heat-exchange blocks are provided through which in each case substreams of this gas are passed.
The invention is particularly suitable in processes in which gas streams which have a pressure of less than 3.5 bar, preferably between 1.1 and 1.8 bar, termed hereinafter low-pressure streams, are to be brought into indirect heat exchange with a heat or cold carrier. According to the invention, in this case, only one of these low-pressure gas streams is conducted through a heat-exchange block, that is to say for each of the gas streams which have a pressure of less than 3.5 bar, a separate heat-exchange block is used.
In the case of gas streams having a pressure greater than approximately 4 bar, the pressure drop in the heat-exchange block plays only a minor role, or can be ignored. Therefore, it is sometimes advantageous to conduct, in addition, such a stream at elevated pressure through at least one of the heat-exchange blocks through which one of the low-pressure gas streams is passed.
The inventive process is used preferably in the low-temperature fractionation of feed air. The gas streams taken off as product from the low-pressure column of a double-column rectifier have only a slight superatmospheric of about 0.1 to 0.8 bar above atmospheric pressure, so that a reduction in pressure drop is of great importance. This applies similarly to gaseous argon product, since the crude argon column is also operated at a relatively low pressure.
Particularly preferably, the gas streams are brought into indirect heat exchange with the feed air. The feed air can be conducted in this case through the heat-exchange blocks in a plurality of streams at different pressure levels. Thus the feed air, on the one hand, can be passed at high pressure-column pressure through the heat-exchange block, for example, and then be fed into the high pressure column, on the other hand the feed air can be recompressed upstream of the heat-exchange block and, after cooling, be work-expanded to produce refrigeration.
In countries having relatively low energy costs decreasing the pressure drops may be of little advantage, since the costs associated with energy saving are high. In these applications it is therefore more expedient not to minimize the pressure drops, but to increase the flow rates, in order to achieve higher pressure drops as a result of which, finally, smaller heat-exchange blocks are required.
Preferably, the gas stream is passed through the heat-exchange block in a manner such that it experiences a pressure drop of 120 to 300 mbar, preferably 120 to 200 mbar. Increasing the pressure drop achieves a greater flow velocity than in the customary heat exchangers, which improves the heat transmission coefficients, which ultimately leads to the fact that the block volume of the heat exchanger can be reduced. For the same pressure drop in the heat-exchange block, the inventive process makes it possible to reduce block volumes by about 15%, compared with the known processes, which results in considerable cost savings.